The present invention relates to an electrical insulator for medium or high voltage, of composite structure, and in particular an insulator for a substation or an electricity line.
As is well known, medium or high voltage electrical insulators are subjected to various stresses, in particular stresses of electrical, mechanical, or thermal origin. If, for whatever reason, these stresses become too high, they run the risk of causing the insulator to fail. It is possible, by visual inspection, to detect and locate insulators that are no longer in good condition when said insulators are built up of quenched glass insulator elements, since under such circumstances the slightest defect gives rise to the faulty insulator element shattering. In contrast, with a composite electrical insulator, a defect can develop without being apparent, for example if it occurs beneath the elastomer covering of the composite insulator. This can continue until the moment when, after a runaway, the insulator is no longer capable of performing its dielectric support function. Such a fault can take the form of an electrical discharge which starts close to one of the metal end-fittings of the insulator and which moves slowly along the support rod of the insulator underneath the insulating covering. This gives rise to slow combustion of the support rod of the insulator, thereby changing the mechanical and dielectric characteristics of the insulator.
The object of the invention is to propose a solution for remedying the above-mentioned drawbacks of composite electrical insulators.
To this end, the invention provides a composite electrical insulator including an optical fiber sensor integrated therein, located inside the insulator. Optical fibers are already used in substation composite insulators for conveying data from one end of the insulator to the other. The invention is based on the fact that an optical fiber can also be used to constitute an integrated sensor for sensing an insulator fault. More particularly, an optical fiber is wound helically on the support rod of the insulator and is in close contact therewith. By selecting an optical fiber having a silica core and optical cladding that melts at a critical temperature, generally below 200xc2x0 C., e.g. optical cladding made of a hard polymer, the beginning of electrical discharges travelling along the support rod of the insulator will cause the temperature of the optical fiber to exceed 250xc2x0 C. locally, thereby causing the optical cladding of the fiber to melt locally and thus damaging the fiber irreversibly. The localized damage to the optical fiber has the effect of attenuating light signals guided in the fiber. A change in the transmission characteristics of the optical fiber can be observed at a measurement unit connected to one end of the fiber to receive the attenuated light signals. The integrated optical fiber fault sensor of the invention can be an optical fiber as mentioned above and that has one of its ends placed inside the insulator and treated so as to act as a reflector, the other end of the fiber being guided outside the insulator for connection to the measurement unit.
In another aspect of the invention, the integrated optical sensor can be a sensor for measuring stresses of mechanical origin, and/or a sensor for measuring stresses of thermal origin acting on the insulator, in particular while it is in service. More precisely, a Bragg grating written in the optical fiber can be used to measure deformation of the support rod of the insulator or indeed to measure temperature levels inside the insulator.
To measure deformation, a Bragg grating is written in a portion of the optical fiber where the protective sheaths have been removed down to the optical cladding. This portion of the fiber which has the Bragg grating written therein is several centimeters long and it is stuck to the support of the insulator, e.g. in such a manner as to extend along the longitudinal axis of the support rod of the insulator so as to be sensitive to longitudinal deformation thereof. The end of the fiber that is guided to the outside of the insulator is connected to a measurement unit suitable for detecting a shift in a spectrum line as reflected by the Bragg grating under the influence of the mechanical stress acting on the insulator. This shift in the spectrum line reflected by the Bragg grating also occurs under the influence of temperature. Adding a second grating along the same optical fiber and subjected to the same temperature variations but not to the same mechanical stresses makes it possible to account for the influence of temperature on the first Bragg grating. It can be preferable for the two Bragg gratings to be centered on different wavelengths so as to ensure that there is no interference between the measurements performed on the two gratings respectively.
If the Bragg grating is used to measure temperature it is written in an end portion of a fiber in close contact with a metal end-fitting of the insulator, e.g. the end-fitting at the high voltage end of the insulator, and it can be used to perform continuous monitoring to ensure that the end-fitting does not heat to a temperature higher than a limit value at which the insulator runs the risk of being damaged. The use of an integrated optical fiber temperature sensor in a composite line electrical insulator of the invention makes it possible, advantageously, to improve the line management facilities of an electricity grid since the sensor can inform the electricity distributor whether or not it is possible to increase the amount of electricity being conveyed without damaging the insulators. Naturally, and without going beyond the ambit of the invention, the Bragg grating could be replaced by some other type of member for measuring stresses of mechanical, thermal, or other origin, intrinsically or extrinsically relative to the optical fiber, but integrated in the insulator.